Download ACTIVITY 1: Forces and Motion

CHAPTER 2
ACTIVITY 1: Forces and Motion
LEARNING TARGET: You will use experimental velocity-­‐time data to identify when a force is acting in the same or opposite direction as its motion. PURPOSE: In Chapter 1 you explored how to describe pushes and pulls in terms of energy transfers between objects. For example, when discussing a soccer player kicking a ball, you may have explained: Energy is transferred from the player’s foot to the ball, causing the ball to increase in speed and kinetic energy. Scientists often use a different way of describing these pushes and pulls, not in terms of energy, but in terms of forces. What evidence suggests that a force is acting on an object in the same or in the opposite direction as its motion? © 2014 PET-­‐hs Page 1 2.1: Forces and Motion INITIAL IDEAS: Complete the following questions individually in your lab notebook: The velocity-­‐time graph below is for a cart that is given a quick tap from a hand and is then released. 1. Draw the velocity-­‐time graph in your laboratory notebook. Circle all the places when you think a force is acting on the cart to affect its motion. Velocity-­‐Time for a Cart 2. Name and describe each force and discuss how each force is affecting the cart’s motion. 3. A student pushes a cart for the entire length of the track. Sketch a velocity-­‐time graph for the motion of the cart from when it was stationary until it reaches the end of the track. Share your ideas with your group members. On a presentation board, record your group’s thoughts about the Initial Ideas. Page 2 2.1: Forces and Motion COLLECTING AND INTERPRETING EVIDENCE: EXPERIMENT #1: WHY DOES A CART CONTINUE TO MOVE AFTER A QUICK TAP? Materials: §
Computer In this simulator we will use force-­‐
time graphs to determine when a force is acting on an object and how the force affects the object’s speed. Force-­‐time graphs show when an object is pushed and the strength of the push. Note: Force is measured in a unit called a Newton (N). Step 1: Go to: http://phet.colorado.edu/en/simulation/forces-­‐1d §
Click the green button Run Now! §
A new window will open with a man and a filing cabinet. §
Click the button Graph Applied Force §
Click the button Graph Velocity §
Take 5 minutes to experiment with the simulator Step 2: On the toolbar on the right of the simulator, (1) uncheck the box Show Total Force, and (2) turn friction off. In this experiment we are just looking at how forces affect motion, we will work with friction in the next experiment. Step 3: Using the computer mouse, have the man push the filing cabinet until it starts to move. After it starts to move quickly let go (so the man stops pushing the filing cabinet), then push the filing cabinet again and quickly let go. Q1. Draw the velocity-­‐time graph and the force-­‐time graph. Page 3 Uncheck This Friction Off 2.1: Forces and Motion Q2. According to the force-­‐time graph, is a force acting on the filing cabinet after the man stopped pushing it? Q3. What happens to the filing cabinet’s speed when the force is acting on it (when the man is actually pushing it)? Q4. What can you say about whether a force is acting on an object by looking at its velocity-­‐time graph? Q5. According to the velocity-­‐time graph, after the man stopped pushing the filing cabinet it continued to move at a constant speed. How can it be that the cabinet continues to move even though there is no force pushing it (try to discuss this using energy ideas)? Step 4: Summarize your understanding from this experiment by responding to the following two questions. Q6. Describe the velocity of an object when a constant force is applied in the same direction it is moving. Use evidence to support your answer. Q7. Describe the velocity of an object when no force is acting on the object. Use evidence to support your answer. Page 4 2.1: Forces and Motion COLLECTING AND INTERPRETING EVIDENCE: EXPERIMENT #2: WHY DO REAL CARTS SLOW DOWN AFTER THE FORCE STOPS? Materials: § Track §
Low-­‐friction cart §
Motion sensor §
Data collection device §
Friction pad Step 1: The motion sensor graph on the right is different from the one you observed in the simulator because after the hand is released, the cart seems to be slowing down slightly. Velocity-­‐Time for a Cart Q1. What do you think caused the cart to slow down? Q2. Use ideas from the previous experiment about how force affects motion to describe why the cart slowed down. Step 2: Place your cart at rest on the track. Start the motion sensor and then give the cart a quick push away from the sensor. After the cart is moving, apply a force in the direction opposite to its motion (apply a gentle force that the cart does not reverse direction). Q3. How did the cart move when a force was applied opposite to the motion? Step 3: Attach a friction pad to the cart so that it is lightly touching the track. Start the motion sensor and again give the cart a quick push away from the sensor. This time, do not use your finger to slow down the cart. Q4. Describe the motion of the cart when the friction pad is used instead of your finger. Q5. What are some similarities and differences about the backward push from your hand and friction? Q6. What evidence from the friction pad velocity-­‐time graph suggests a force is acting on an object in the opposite direction it is moving? Q7. Would you say that friction is a force? What evidence would you use to support your answer? Page 5 2.1: Forces and Motion COLLECTING AND INTERPRETING EVIDENCE: EXPERIMENT #3: WHAT HAPPENS TO A CART’S SPEED WHEN A CONSTANT FORCE IS CONTINUOUSLY APPLIED? Materials: §
Track §
Low-­‐friction cart §
Fan §
Motion sensor §
Data collection device Step 1: Place the cart near one end of the track (or table). Using one finger, push the cart (on the back of the cart and do not push down on the cart) along the track. Keep your finger touching the cart the whole time and try to maintain a constant strength push as the cart moves. Each group member must try this. Q1. How does the cart move when you push it? Q2. Were you able to exert a constant strength push on the cart the whole time? Step 2: You probably noticed it is challenging for your finger to keep up with the cart and apply constant strength force. For the next steps, you will use a fan to apply a constant strength force to the cart. Attach the fan to the cart. Step 3: Place the cart on the track in front of the motion sensor so the fan pushes the cart away from the sensor. Begin collecting velocity-­‐time data and then turn on the fan and let go of the cart (do not push or pull the cart). Draw a velocity-­‐time graph for the motion of the cart. Q3. What evidence from the velocity-­‐time graph suggests a force is continuously acting on an object in the direction it is moving? Page 6 2.1: Forces and Motion Step 4: Look back to your initial idea about a student who pushes a cart for the entire length of the track. Q4. What did you think the velocity-­‐time graph would look like? Q5. Explain how your Initial Ideas changed and what evidence caused you to change your thinking. Page 7 2.1: Forces and Motion SUMMARIZING QUESTIONS Name: _______________________________ What evidence suggests that a force is acting on an object in the same or in the opposite direction as its motion? LEARNING TARGET: You will use experimental velocity-­‐time data to identify when a force is acting in the same or opposite direction as its motion. 1. On the graph axes (on the right), draw two graphs: a. Of a hockey puck that is pushed by a quick force (applied only for 1 second). b. Of a hockey puck that is continuously pushed by a constant-­‐strength force (for the entire time of the graph). c. Describe how and why the two graphs are different. 2. What evidence suggests: a. A force is acting in the same direction that an object is moving? b. A force is acting in the opposite direction that an object is moving? c. Friction is a force? 3. Suppose that while a cart was being pushed along the track by the fan, a wire suddenly broke and the fan stopped spinning. What do you think would happen to the speed of the cart? Explain your reasoning (a graph may be helpful). Page 8 2.1: Forces and Motion 4. For the velocity-­‐time graph (shown below), draw a force-­‐time graph for the cart. Hint: make sure to pay attention to the time when you think the force is acting. 5. Three students were discussing an experiment where a cart was given a quick tap and then the hand was removed. They are discussing why they think the cart continues to move after it was released from the hand. Read their comments. After the hand let go, the force of the hand is transferred to the cart and is carried with it. That’s why the cart keeps moving. Samantha After the hand let go, the force of the hand is no longer acting on the cart. That’s why it moves differently. Amara The force of the hand stops when it lets go, but some other force m ust take over to keep the cart moving. Victor How would you respond to these students in the discussion? Be sure to include evidence. 6. Describe why objects slow down due to friction in terms of both energy transfer and force. Page 9 2.1: Forces and Motion 2.1 SCIENTIST IDEAS READING Instructions: Read the Scientist Ideas, paying careful attention to each key idea. When you read, try to think about how the key ideas relate to the evidence you collected in the activity. Forces Cause Change in Speed Idea: The evidence that a force is acting on an object is that the object will change speed. No Force, No Motion Idea: Without a force acting on an object, the object will not change speed. This means that the object will either remain stationary or continue to move at a constant speed. When a force acts on a stationary object, it will start moving. A force that acts in the same direction as an object’s motion will cause the object to speed up. When a force acts in the direction opposite to the motion of an object, the object to slow down. Scientists summarize the motion of objects without any forces acting in the following ways: §
§
When no forces act on a stationary object, it will stay stationary. When no forces act on a moving object it will continue to move at a constant speed. Force Diagram Idea: Force diagrams use arrows to represent the forces acting on an object. The arrows show both the strength of the force and the direction it was applied. Just like with the energy diagrams, we can also draw a diagram to help us represent forces. They are labeled to show where the force came from (for example Fpush). Notice that the force diagram shows only one of the objects involved in the interaction (the cart), and that the object applying the force (the hand or the other cart) is only mentioned in the label of the force arrow. Page 10 2.1: Forces and Motion Friction Force Idea: When no other forces are acting on an object, friction will cause the object to slow down. This is evidence that friction is a force. Since friction causes objects to slow down, scientists have concluded that it is a force that acts in the opposite direction as an object’s motion. In the case of friction force (F Friction), the force diagram is always drawn with the force in the opposite direction as the motion of the object. It is challenging to think about a scenario where friction is completely removed because as long as two objects are in contact, friction will be present. If it were possible to remove frictional forces once an object is already in motion, the object would move at a constant speed (after an initial push). Remember that scientists use the term negligible to indicate that friction is present, but it is so small in comparison to the other pushes and pulls that it can be ignored. Force-­‐Time Graph Idea: Force-­‐time graphs express the time intervals when a force is acting on an object. Energy Transfers when Force is Applied Idea: When a force acts on an object, energy is either transferred into or out of the object. The force-­‐time and velocity-­‐time graphs (on the right) confirm the idea that the force was only applied during the time intervals when the speed of the cart increased. This idea can also be thought about in terms of energy: §
When a force acts in the same direction an object is moving, the object increases in speed and energy is transferred into the object (the object receives energy). §
When a force acts in the opposite direction an object is moving, the object decreases in speed and energy is transferred out of the object (the object gives energy). Think about a stationary cart that is given a quick push with a hand and starts moving. Shown on the next page are a velocity-­‐
time graph and a force-­‐time graph: before the interaction, during the interaction, and after the interaction. Force and energy diagrams for each of these times are shown. Page 11 2.1: Forces and Motion Force and Energy Diagrams Friction can also be explained both in terms of energy and in terms of force: In terms of energy: with friction, energy will be transferred out of the moving object, so the object will decrease in kinetic energy and increase in thermal energy (and therefore will slow down). In terms of force: an object will slow down when a force is applied in the opposite direction as motion; therefore when a friction is present, an object will slow down. Page 12 2.1: Forces and Motion Respond to the following questions individually in your lab notebook: 1. Summarize how forces affect an object’s speed by completing the statements below: When a force acts on a stationary cart, the cart will… When a force acts on a moving cart, the cart will… When no forces act on a stationary cart, the cart will… When no forces act on a moving cart, the cart will… 2. Draw a force-­‐time graph for the velocity-­‐time graph that is shown on the right. 3. Is friction considered a force? Explain why or why not using evidence. 4. Use the force diagrams below to indicate whether the cart is not moving, moving at a constant speed, speeding up, or slowing down. a. d. F Hand
F Pull
b. e. F Pull
c. f. F Hand Push
5. Draw a force diagram for a cart moving to the right that is given a push in the same direction it is moving. Label the force arrow. 6. Draw a force diagram for a stationary cart that is hit from the left by a moving cart. Label the force arrow. 7. Think about a moving cart (launched cart) that hits a stationary cart (target cart). For this energy transfer and force, complete the following: a. A velocity-­‐time graph for the target cart b. A force-­‐time graph for the target cart c. An energy diagram for the energy transfer between the two carts d. A force diagram for the launched cart e. A force diagram for the target cart 8. Is energy transferred during, or after the time when the force is applied (or a combination of these)? Explain your thinking. 9. When a force acts on the cart in the same direction as its motion, is the cart the energy giver or the energy receiver? Explain using evidence. 10. When a force acts on the cart in the opposite direction to its motion, is the cart the energy giver or the energy receiver? Explain using evidence. Page 13 2.1: Forces and Motion 2.1 PRACTICE QUESTIONS Instructions: Complete the practice problems in your laboratory notebook. Work individually, and then check your responses with your group. 1. Copy the velocity-­‐time and force-­‐time graphs (on the right) into your laboratory notebook. a. Circle each part on the velocity-­‐time graph that show a force was acting on the cart. b. Describe how you are able to tell if a force is acting on an object by looking at the velocity-­‐time graph. c. Describe how you are able to tell if no forces are acting on an object by looking at the velocity-­‐time graph. d. Describe how the force-­‐time graph supports your ideas from questions b and c (hint: use the time intervals to help you). 2. If an object is already moving at a constant speed, what effect on its speed does a force applied in the direction opposite its motion have? If this force keeps acting on the object, what else may happen? 3. Shown below are four force arrow diagrams. For each force arrow diagram, describe the motion of the cart (both speed and direction). a. d. F Tap
F Hand
b. e. F Friction
c. f. F cart 4. Draw a force diagram for a soccerball that is kicked by a player. Label the force arrow. 5. Draw a force diagram for the soccerball coming to a stop because of friction between the ball and the grass. Label the force arrow. 6. Draw a force diagram for a stationary cart that is hit from the left by a moving cart. Label the force arrow. Page 14 2.1: Forces and Motion 7. Victor and Amara are discussing what will happen to a cart when it is given a short push. If a cart is pushed for a second, the force is only applied If a cart is pushed for a second it will speed up for a while, for a second. The cart will only slow down if another but it must slow down eventually because there is only so acts on it. force much force in the cart. Victor Amara Do you agree with Victor or Amara? Choose one and explain why you agree with him/her and why you disagree with the other. 8. Dave and Luisa are discussing their ideas about the similarities and differences between energy and force. I don’t think they are the same. I think that the force is only there while the two objects are in contact and that’s when the energy is transferred. When there is not contact, there is no force, and so no energy transfer. Force is never transferred! I think energy and force are both the same. When objects are in contact, both energy and force are transferred from one object to another. So, after the interaction one object has more energy and force than it had before, and the other has energy and force. less Dave Luisa Do you agree with Dave, Luisa, or neither of them? Explain your thinking. 9. Suppose a spacecraft is stationary in deep space (far from any stars or planets, so that all external forces like friction or gravity are negligible). The spacecraft’s rocket is fired for a period of 2 seconds and then is shut off. a. What do you think the motion of the spacecraft would be like after the engine is shut off? Explain your reasoning. b. Draw a velocity-­‐time graph for the motion of the spacecraft from the moment the rocket is started until long after the engine was turned off. c. Draw two force diagrams for the spacecraft. While the engine is on Page 15 After the engine is turned off